Trim tabs are hydrodynamic surfaces that are mounted at the aft of a motor boat near its chine's. Trim tabs are generally comprised of a rigid material and have been commercially available for many years and are generally standard equipment on most large power boats.
The position of trim tabs is often adjusted via a hydraulic or electric ram, controlled by either a joystick type switch, or rocker switches mounted at the helm. Trim tabs deflect a portion of the water passing under the hull of the vessel, changing the attitude of the hull in the water. The effect of the trim tabs is most pronounced in the pitch (fore and aft) attitude about a lateral axis, and secondly in the roll (side to side) attitude about a longitudinal axis. Trim tabs are commonly used to correct lists (roll of the boat about the longitudinal axis), and to correct ‘bow up’ conditions about the lateral axis but the average boater has little knowledge as to the effect this may have on their boats fuel economy or speed.
A boat hull passing through the water generates a fair amount of drag. This drag is responsible for the notoriously bad fuel mileage experienced by most power boats. Planning boat hulls are designed for a particular attitude, both in pitch and roll. By adding weight (provisions, dinghy, water, fuel, etc.) boat operators may adversely affect the factory engineered balance of the boat. A boat traveling at the attitude for which it was designed will be traveling at its most efficient attitude. Changes in water conditions, weather, and currents can alter the attitude at which the boat is most efficient. Running weight of the boat changes when passengers move about or when fuel is consumed the boats optimum attitude is affected. Because the attitude of a boat is influenced by so many factors, boat manufacturers have opted to equip boats with trim tabs, to allow operators to correct for these factors affecting boat trim.
Most operators do not monitor the boat's attitude often enough. Changing conditions can require the operator make adjustments to the trim tab positions on a continual basis. To expect a boat operator to be this vigilant to the trim tabs is simply not reasonable.
Most boat operators trim out their boat's attitude either by closely watching the vessel speed, or by simple gut instinct. Conventional wisdom dictates that for a given throttle setting, the most efficient attitude will also be the maximum vessel speed. Typically, the operator may make a small manual change to the trim tab settings, and monitor the speed of the boat. If the speed goes up, the operator may make another adjustment of the tabs, in the same direction. If the speed decreases, he might make an adjustment in the opposite direction. This process is repeated until the operator is sufficiently convinced he has achieved the best possible speed out of his boat for the given throttle position and prevailing conditions. The entire process is continually repeated due to changing water, load, and weather conditions. Of course this manual adjustment process is time consuming by the operator and given the small velocity changes of the boat and scalable variations in the time lag to acquire actionable data, the manual process is fraught with error.
Trim tabs are normally positioned relative the boat on which they are mounted by hydraulic power or by direct driven screw geared rams. Trim tabs are variable in position so as to change the attitude of the hull of a moving boat with respect to a horizontal axis and about a longitudinal axis if only one of the tabs is altered.
Trim tabs provide several useful functions as a result of being able to change the attitude of a moving boat's hull. These include (i) increased speed, (ii) improved fuel economy and reduced laboring of the boat's engines, (iii) improved forward visibility, (iv) reduction of pounding, listing, squatting, and/or wake, (v) adjustment of the boat's attitude to a position that is safer or more comfortable to the boat's occupants, (vi) minimization of bow rise when the boat comes up on plane, (vii) reduction in time and energy for the boat to reach its planning speed.
The aforementioned benefits to the boat's operation generally accrue only when the trim tabs are in the proper position. The latitude, or range, of the proper position of the trim tabs varies from boat to boat, and from time to time. The range of “proper position” may be as critical as plus or minus fractions of degrees. Meanwhile, trim tabs are typically variable through a range of greater than 25 to 40 degrees depending on the size and manufacturer. When the trim tabs are in a position that deviates greatly from optimal then they may actually serve to aggravate one or more operational problem conditions of the boat. Even when the trim tabs are positioned close or very close to optimal, the boat's speed and fuel economy may nonetheless be compromised a few percentage points from what the boat could achieve if the trim tabs are more precisely optimally positioned. Further, the trim tabs should be dynamically positioned during various operational conditions and uses of the boat.
The required control of a power boat's trim tabs may be, at times and from time to time, complex. Nonetheless to address this complexity, existing automated power trim tab control systems simply cause a boat to assume, and hold a particular attitude both fore to aft and port to starboard. Therefore more sophisticated trim control is desired to provide more functionality than simply attitudinal-position-holding.
Most marine vessels are designed so that when they are at rest in the water, the deck generally lies at a small angle (2°-4°) from the horizontal with the bow slightly higher than the stern. On power craft with planning hulls, the boat will lift out of the water as the boat gains speed. If the drive system of the boat is located under and slightly behind its center of mass, the boat will rise almost vertically out of the water, maintaining a near horizontal attitude, and the helmsman will maintain good visibility of the water ahead. However, if the center of mass of the craft is well astern of the center of the hull, then the bow will typically rise from five to thirty degrees (5°-30°) out of the water as the boat comes up on plane.
A boat with extreme bow rise obstructs the helmsman's visibility of the water ahead as the boat comes up on plane. As the boat gains speed, more and more of its bow comes out of the water until the center of gravity of the boat begins to break out of the water. At this point the boat's hull falls to its full on-plane condition. The on-plane boat typically rides from two to eight degrees (2°-8°) from the horizontal. The precise angle the boat assumes is dependent upon the magnitude and distribution of its load, and on its hull design.
A given hull design begins to plane at a predetermined speed. When the boat slows down it will go off-plane at a slightly lower speed than the speed required to place it on plane. Therefore, automatic trimming devices should take this hysteresis effect into account when implementing the automatic trimming devices. In sum, with boats there is always an “off-plane to on-plane” speed and a slightly lower “on-plane to off-plane” speed.
The present manufacturers of manually controlled power trim tabs recommend the following procedures for the proper adjustment of the trim tabs.
As the vessel accelerates, the bow will tend to rise out of the water, reducing visibility. To correct the obstructed visibility problem, the trim tabs are moved downwardly. Once the vessel is on plane, the tabs may be adjusted for optimal performance and trim (Bennett™ provides a trim tab usage demo, available on the Bennett™ trim tab web site).
Because boats and more specifically large boats such as cruisers and yachts consume fuel typically derived from scarce petroleum presently at increasing prices, it is useful to implement a method for electronically controlling boat's power trim tabs in order to improve the fuel economy of the boat.
Additionally, since large boats tend to be moored, and not often taken out of the water, it is advantageous to raise the trim tabs to their fully retracted position when the vessel is not in use. Raising the trim tabs reduces the amount of trim tab components exposed to the harsh marine environment. Similarly, on boats that are regularly removed from the water, it is advantageous to fully retract the trim tabs to prevent exposure to damage from external sources (trailers and lifts), and to protect the internal components of the trim tab rams from the environment.